Rear-focus zoom lens system

ABSTRACT

A rear-focus zoom lens system with four lens group. A stationary first lens group of a positive refractive power; a second lens group of a negative refractive power that moves for zooming; a stationary third lens group of a positive refractive power; a fourth lens group of a positive refractive power that moves for zooming and focusing. The first through fourth lens groups are arranged in that order from the object side. This zoom lens system also satisfies the following conditional expression: 
     
         3.7&lt;f.sub.4 /fw&lt;6 
    
     wherein f 4  is the focal length of the fourth lens group and fw is the focal length of the zoom lens system at the wide-angle end.

This application is a continuation of application Ser. No. 07/977,024 filed Nov. 16, 1992, now abandoned.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a rear-focus zoom lens system with a variable power ratio of about 12 and a brightness, or F value, approximately in the range from 1.6 to 1.8 and, more particularly, to a compact-size high variable power zoom lens system having a large view angle, i.e., 65° or greater. This zoom lens system is useful for a video camera or a still camera.

2. Description of the Related Art

As the size and weight of video cameras and still cameras are being reduced, there is a growing need for smaller and lighter lenses. In response to this need, a variety of systems have been proposed. Some of the systems use the conventional, and so-called front lens focus method, which performs focusing by moving the first lens group, the first lens group being the closest to an object and larger than the other lens groups. Some other systems use a so-called rear focus method, which performs focusing by moving a lens group other than the first lens group.

Generally, because a zoom lens system of the rear focus type moves a relatively small and light-weight lens group to perform focusing, that method reduces the required driving force and provides for a speedy focusing.

For example, the assignee of the present invention has proposed, in Japanese Patent Applications Laid-Open Nos. 62-24213 and 62-215225, a zoom lens system of the rear focus type which comprises: a stationary first lens group; a movable second lens group for varying the magnification; a stationary third lens group having a positive refractive power; and a movable fourth lens group having a positive refractive power. In that combination, the second lens group moves to vary magnifications and the fourth lens group moves to compensate for changes in the image field caused by magnification variations and also to perform focusing.

In order to reduce the size of the first lens group, in both a front lens focus type and a rear focus type, the focal length at the wide-angle end (which determines the size of the first lens group) typically is made as close to the telephoto end as possible. Therefore, in most lens systems of these types, the view angle at the wide-angle end is generally 50° or so. A lens system having a view angle of the wide-angle end greater than 50° inevitably has required a complicated lens system, which would not meet the need for a compact size.

To obtain a view angle of 65° or greater in a video camera, either a wide angle converter lens or a wide angle attachment lens typically has been attached in front of the lens system, which is a troublesome operation.

SUMMARY OF THE INVENTION

Accordingly, an object of the present invention is to provide an improved rear-focus zoom lens system which has a view angle at the wide-angle end of 65° or greater and a high variable power ratio and achieves good optical performance.

This rear-focus lens system has a first lens group having a positive refractive power that remains fixed during zooming; a second lens group having a negative refractive power that moves for zooming; a third lens group having a positive refractive power that remains fixed during zooming; and a fourth lens group of positive refractive power that moves for zooming and focusing. The first through fourth lens groups are arranged in that order from the object side. This zoom lens system also satisfies the following conditional expression:

    3.7<f.sub.4 /fw<6

wherein f₄ is the focal length of the fourth lens group and fw is the focal length of the zoom lens system at the wide-angle end.

Further objects, features and advantages of the present invention will become apparent from the following description of the preferred embodiments with reference to the attached drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 shows a sectional view of a lens system and the aberrations thereof, according to a first embodiment of the present invention.

FIG. 2 shows a sectional view of a lens system and the aberrations thereof, according to a second embodiment of the present invention.

FIG. 3 shows a sectional view of a lens system and the aberrations thereof, according to a third embodiment of the present invention.

FIG. 4 shows a sectional view of a lens system and the aberrations thereof, according to a fourth embodiment of the present invention.

FIG. 5 shows a sectional view of a lens system and the aberrations thereof, according to a fifth embodiment of the present invention.

FIG. 6 shows a sectional view of a lens system and the aberrations thereof, according to a sixth embodiment of the present invention.

FIG. 7 shows a sectional view of a lens system and the aberrations thereof, according to a seventh embodiment of the present invention.

FIG. 8 shows a sectional view of a lens system and the aberrations thereof, according to an eighth embodiment of the present invention.

The section (A) of each figure shows a sectional view of the lens system, and the sections (B) and (C) of each figure show the aberrations at the wide-angle end and the aberrations at the telephoto end, respectively.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

Zoom lens systems according to the preferred embodiments of the present invention will be described hereinafter, with reference to the accompanying drawings.

Referring to section (A) of each figure, a lens system according to the present invention comprises: a first lens group I having a positive refractive power, a second lens group II having a negative refractive power, a third lens group III having a positive refractive power, a fourth lens group IV having a positive refractive power, and a stop SP. These groups are arranged in that order from the object side. The first and third lens groups I, III are fixed in position. The second and fourth lens groups II, IV are movable for zooming, and the fourth lens group IV also is movable for focusing.

In order to obtain a large view angle at the wide-angle end, of about 65° or greater, while still reducing the total length of the lens system and achieving good aberration correction, the zoom lens system satisfies the following conditional expression:

    3.7<f.sub.4 /fw<6                                          (1)

wherein f₄ is the focal length of the fourth lens group and fw is the focal length of the entire lens system at a wide-angle end.

The conditional expression (1) defines the focal length of the fourth lens group. If this expression is satisfied, both a short length of the lens system and small changes in the aberrations caused by a displacement of the fourth lens group for focusing will be achieved. The light incident on the fourth lens group is made substantially afocal by the lens groups in front thereof. If f₄ /fw is 6 or greater, the length of the lens system becomes unacceptably large. If f₄ /fw is 3.7 or less, changes in aberrations caused by a displacement of the fourth lens group for focusing becomes unacceptably large, and it will be difficult to achieve good optical performance over the entire range of object distances.

Preferably, the zoom lens system also should satisfy the following conditional expressions:

    2.8<|M.sub.2 /f.sub.2 |<5                (2)

    7<f.sub.1 /fw<15                                           (3)

wherein M2 is a displacement of the second lens group when the zoom lens moves from the wide-angle end to the telephoto end, and fi is the focal length of the ith lens group.

The conditional expression (2) states the conditions for achieving an appropriately high variable power ratio while retaining good optical performance. If |M₂ /f₂ | is 5 or greater, the displacement of the second lens group is too large to achieve a compact lens system, or the focal length of the second lens group is unfavorably small so that the Petzval's sum becomes too large in negative numbers and the image field becomes overly large. If |M₂ /f₂ | is 2.8 or less, it will be difficult to obtain the high variable power ratio zoom lens that is desired.

The conditional expression (3) states the conditions for mitigating aberrations, particularly comatic aberration, so as to achieve good imaging over the entire range of zoom lens variable powers. If f₁ /fw is 15 or greater, a good view angle characteristic can be obtained and the changes in the aberrations, such as comatic aberration, can be mitigated. However, the diameter of the first lens group must be made substantially large in order to achieve a wide view angle of 65° or greater, which is undesirable for achieving a compact lens system. If f₁ /fw is 7 or less, the light fluxes around the lenses near the wide-angle end will cause an outgoing coma and, therefore, large changes in the aberrations.

The conditional expression (3) states the conditions for achieving an appropriate balance between a wider view angle and a smaller size of the lens system. To make the balance more appropriate, i.e., to achieve a more advantageous zoom lens system, the range of f₁ /fw may be narrowed as follows:

    8<f.sub.1 /fw<12

As mentioned above, the outside diameter of the first lens group must be made large in order to achieve a wide view angle. To provide a sufficiently thick peripheral portion in a positive lens of the first lens group having a large diameter, the positive lens must have a substantially thick central portion, which makes the production of the lens difficult. To avoid such a problem, the first lens group should preferably be composed of two positive lens and a combined lens made of a positive lens and a negative lens.

Further, to enhance the optical performance of the central portion of the lens system, the third lens group should preferably comprise an aspherical lens, which reduces spherical aberration and helps achieve a larger maximum aperture, and lower F number.

To fully achieve the objects of the present invention, the following conditional expressions also should preferably be satisfied.

    4.0<|f.sub.1 /f.sub.2 |<10.0             (4)

    1.55<|f.sub.1 /fw|<1.8                   (5)

    1.0<f.sub.1 /M.sub.2 <1.85                                 (6)

    1.0<z/z.sub.2 <1.5                                         (7)

    1.65<z.sub.3 /z.sub.4 <4.0                                 (8)

wherein z_(i) is the ratio between the magnifications at the wide-angle end and the telephoto end in the ith lens group (the ratio between paraxial lateral magnifications); and z is f_(T) /fw (f_(T) representing the focal length of the entire lens system at the telephoto end).

The conditional expressions (4) to (6) state the conditions that help to achieve a small lens system as well as good optical performance.

If the conditional expression (4) is satisfied, smooth magnification changing and reduced comatic aberration can be achieved in a small lens system. If |f₁ /f₂ | is 10.0 or greater, the comatic aberration near the wide-angle end is reduced. However, the lens system then inevitably becomes large, and changes in the aberrations, particularly a change in the curvature of image field, caused by zooming will become large. If |f₁ /f₂ | is 4.0 or less, the entire zoom lens system must be made quite large, in most cases, in order to achieve the desired high variable power ratio.

If the conditional expression (5) is satisfied, Petzval's sum is mitigated in the second lens group, which is the main variable-power lens system. If |f₁ /fw| is out of the defined range, a substantial curvature of the image field will likely result, and thus good imaging will become hard to achieve.

The conditional expression (6) states a condition which defines the displacements of the second lens group, i.e., the main variable-power lens system that is moved for zooming. If this conditional expression is satisfied, a reduced diameter of the first lens group is achieved and the occurrence of comatic aberration is suppressed. If f₁ /M₂ is 1.85 or greater, the diameter of the front lens becomes unacceptably large. If f₁ /M₂ is 1.0 or less, the comatic aberration, particularly near the wide-angle end, becomes large.

If the conditional expressions (7) and (8) are satisfied, an appropriate balance among the variable powers of the lens groups, as well as good optical performance, can be achieved in a small lens system. If z/z₂ and/or z₃ /z₄ are out of the respective ranges, good optical performance cannot be achieved. Further, the size of the lens system or changes in the image field caused by variable power becomes unacceptably large.

In a preferred zoom lens system according to the present invention, the first lens group consists of a combined lens made of negative and positive lenses and one or two positive lenses placed behind the combined lens; the second lens group consists of a negative meniscus lens having a substantially concave surface toward the image side, and a biconcave lens and a positive lens having a concave surface toward the image side. These lenses are arranged in that order, from the object side. The third lens group consists of at least one positive lens and at least one negative lens.

Further, the third lens group should preferably be composed of a biconvex positive lens and a negative lens having a concave surface toward the object side, which also are arranged in that order from the object side. The above-mentioned aspherical surface should preferably be present on this biconvex lens for an advantageous correction of spherical aberration.

The fourth lens group should preferably consist of at least one positive lens and at least one negative lens. Since the light incident on the fourth lens is substantially afocal in most cases, the focal length of the fourth lens group should preferably be arranged symmetrically to the focal length of the third lens group. More preferably, the fourth lens group should consist of a negative meniscus lens having its concave surface toward the image side and one or two positive lenses, which are arranged in that order from the object side. Such a fourth lens group provides a good focusing characteristic.

The preferred embodiments of the present invention will be described below, by showing numerical values of various lens factors. In the examples: r_(i) is the radius of curvature of the lens surface located in the ith position in order from the object side; d_(i) is the thickness of the ith glass or air lens in order from the object side; n_(i) is the refractance of the ith glass lens in order from the object side; and v_(i) is the Abbe number of the ith glass lens.

If the X axis is assumed along the optical axis; the Y axis is assumed perpendicular to the optical axis; and the origin is assumed on the intersecting point between the X axis and a straight line passing the apexes of the lens, the aspherical surface is expressed as follows:

    X={(1/R)Y.sup.2 /(1+√1-(Y/R).sup.2 }+a.sub.1 Y.sup.2 +a.sub.2 Y.sup.4 +a.sub.3 Y.sup.16 +a.sub.4 Y.sup.8 +a.sub.5 Y.sup.10 +b.sub.1 Y.sup.3 +b.sub.2 Y.sup.5 +b.sub.3 Y.sup.7 +b.sub.4 Y.sup.9

where: R is the paraxial radius of curvature of the lens surface; a₁, a₂, a₃, a₄, a₅, b₁, b₂, b₃, and b₄ each are coefficients of the aspherical surface.

In the below expressions showing the values a₁ to a₅, D₋₀₃, for example, means 10⁻³.

Parallel flat glass plate typically is placed at the very front, i.e., the side closest to an object, as a face plate, filter or the like.

The figures corresponding to the above conditional expressions are shown in Table 1 below.

    ______________________________________                                         Embodiment 1                                                                   f = 1 to 11.41   fno = 1:1.85 to 1.93   2w = 66.3° to                   ______________________________________                                         6.6°                                                                    r 1 = 12.2800                                                                             d 1 = 0.3061                                                                               n 1 = 1.84666                                                                              ν 1 = 23.8                               r 2 = 7.4755                                                                              d 2 = 2.1409                                                                               n 2 = 1.51633                                                                              ν 2 = 64.2                               r 3 = -198.4315                                                                           d 3 = 0.0408                                                        r 4 = 6.6504                                                                              d 4 = 1.2589                                                                               n 3 = 1.69680                                                                              ν 3 = 55.5                               r 5 = 16.3819                                                                             d 5 = variable                                                      r 6 = 12.8170                                                                             d 6 = 0.1633                                                                               n 4 = 1.77250                                                                              ν 4 = 49.6                               r 7 = 1.8119                                                                              d 7 = 1.0857                                                        r 8 = -6.6579                                                                             d 8 = 0.1429                                                                               n 5 = 1.69680                                                                              ν 5 = 55.5                               r 9 = 2.6709                                                                              d 9 = 0.2500                                                        r10 = 2.9732                                                                              d10 = 0.6710                                                                               n 6 = 1.84666                                                                              ν 6 = 23.8                               r11 = 10.4334                                                                             d11 = variable                                                      r12 = (stop)                                                                              d12 = 0.3673                                                        r13 = 5.0973                                                                              d13 = 0.6077                                                                               n 7 = 1.69680                                                                              ν 7 = 55.5                               r14 = -6.6462                                                                             d14 = 0.1442                                                        r15 = -3.0100                                                                             d15 = 0.2041                                                                               n 8 = 1.68893                                                                              ν 8 = 31.1                               r16 = -4.0610                                                                             d16 = variable                                                      r17 = 6.2677                                                                              d17 = 0.1633                                                                               n 9 = 1.78472                                                                              ν 9 = 25.7                               r18 = 2.1426                                                                              d18 = 0.5836                                                                               n10 = 1.51633                                                                              ν10 = 64.2                               r19 = -10.4932                                                                            d19 = 0.0306                                                        r20 = 3.6185                                                                              d20 = 0.4724                                                                               n11 = 1.51633                                                                              ν11 = 64.2                               r21 = -14.8477                                                                            d21 = variable                                                      r22 = ∞                                                                             d22 = 1.0204                                                                               n12 = 1.51633                                                                              ν12 = 64.2                               r23 = ∞                                                                  ______________________________________                                         Variable Thicknesses of Air Lenses                                                       Focal Lengths                                                                    W            M      T                                              Air Lenses  1.00         1.81   11.41                                          ______________________________________                                         d 5         0.21         2.44   6.42                                           d11         6.73         4.49   0.52                                           d16         1.94         1.58   1.37                                           d21         1.02         1.38   1.59                                           ______________________________________                                    

    ______________________________________                                         Embodiment 2                                                                   f = 1 to 11.41   fno = 1:1.85 to 1.92   2w = 66.3° to                   ______________________________________                                         6.6°                                                                    r 1 = 12.8564                                                                            d 1 = 0.3061                                                                               n 1 = 1.84666                                                                               ν 1 = 23.8                               r 2 = 7.2757                                                                             d 2 = 2.0610                                                                               n 2 = 1.51633                                                                               ν 2 = 64.2                               r 3 = -58.2980                                                                           d 3 = 0.0408                                                         r 4 = 6.2666                                                                             d 4 = 1.1215                                                                               n 3 = 1.69680                                                                               ν 3 = 55.5                               r 5 = 16.7190                                                                            d 5 = variable                                                       r 6 = 17.3934                                                                            d 6 = 0.1633                                                                               n 4 = 1.77250                                                                               ν 4 = 49.6                               r 7 = 1.6562                                                                             d 7 = 0.8526                                                         r 8 = -5.6964                                                                            d 8 = 0.1429                                                                               n 5 = 1.69680                                                                               ν 5 = 55.5                               r 9 = 2.7650                                                                             d 9 = 0.2205                                                         r10 = 2.9381                                                                             d10 = 0.4612                                                                               n 6 = 1.84666                                                                               ν 6 = 23.8                               r11 = 10.4334                                                                            d11 = variable                                                       r12 = (stop)                                                                             d12 = 0.3673                                                         r13 = 5.1418                                                                             d13 = 0.6262                                                                               n 7 = 1.69680                                                                               ν 7 = 55.5                               r14 = -5.9596                                                                            d14 = 0.2272                                                         r15 = -3.0337                                                                            d15 = 0.2041                                                                               n 8 = 1.68893                                                                               ν 8 = 31.1                               r16 = -4.4716                                                                            d16 = variable                                                       r17 = 6.7044                                                                             d17 = 0.1633                                                                               n 9 = 1.78472                                                                               ν 9 = 25.7                               r18 = 2.1807                                                                             d18 = 0.5904                                                                               n10 = 1.51633                                                                               ν10 = 64.2                               r19 = -8.6645                                                                            d19 = 0.0306                                                         r20 = 3.7745                                                                             d20 = 0.6083                                                                               n11 = 1.51633                                                                               ν11 = 64.2                               r21 = -12.8287                                                                           d21 = variable                                                       r22 = ∞                                                                            d22 = 1.0204                                                                               n12 = 1.51633                                                                               ν12 = 64.2                               r23 = ∞                                                                  ______________________________________                                         Variable Thicknesses of Air Lenses                                                       Focal Lengths                                                                    W            M      T                                              Air Lenses  1.00         2.03   11.41                                          ______________________________________                                         d 5         0.19         2.63   6.00                                           d11         6.45         4.01   0.65                                           d16         1.94         1.49   1.41                                           d21         1.02         1.47   1.54                                           ______________________________________                                    

    ______________________________________                                         Embodiment 3                                                                   f = 1 to 11.41   fno = 1:1.85 to 1.89   2w = 66.3° to                   ______________________________________                                         6.6°                                                                    r 1 = 23.4043                                                                             d 1 = 0.3061                                                                               n 1 = 1.84666                                                                              ν 1 = 23.8                               r 2 = 7.7547                                                                              d 2 = 1.1701                                                                               n 2 = 1.51633                                                                              ν 2 = 64.2                               r 3 = 87.2489                                                                             d 3 = 0.0408                                                        r 4 = 17.5012                                                                             d 4 = 0.6360                                                                               n 3 = 1.72000                                                                              ν 3 = 50.3                               r 5 = -182.0458                                                                           d 5 = 0.0408                                                        r 6 = 5.7152                                                                              d 6 = 0.8453                                                                               n 4 = 1.72000                                                                              ν 4 = 50.3                               r 7 = 17.1471                                                                             d 7 = variable                                                      r 8 = 14.2365                                                                             d 8 = 0.1633                                                                               n 5 = 1.77250                                                                              ν 5 = 49.6                               r 9 = 1.4683                                                                              d 9 = 0.7496                                                        r10 = -3.9438                                                                             d10 = 0.1429                                                                               n 6 = 1.69680                                                                              ν 6 = 55.5                               r11 = 3.1284                                                                              d11 = 0.2050                                                        r12 = 3.0315                                                                              d12 = 0.3856                                                                               n 7 = 1.84666                                                                              ν 7 = 23.8                               r13 = 26.2836                                                                             d13 = variable                                                      r14 = (stop)                                                                              d14 = 0.2449                                                        r15 = 4.2460                                                                              d15 = 0.5749                                                                               n 8 = 1.58267                                                                              ν 8 = 46.4                               r16 = -4.2460                                                                             d16 = 0.0963                                                        r17 = -2.6769                                                                             d17 = 0.2041                                                                               n 9 = 1.68893                                                                              ν 9 = 31.1                               r18 = -4.0729                                                                             d18 = variable                                                      r19 = 5.8421                                                                              d19 = 0.1633                                                                               n10 = 1.84666                                                                              ν10 = 23.8                               r20 = 2.1729                                                                              d20 = 0.5658                                                                               n11 = 1.51633                                                                              ν11 = 64.2                               r21 = -6.1810                                                                             d21 = 0.0306                                                        r22 = 3.6317                                                                              d22 = 0.4249                                                                               n12 = 1.48749                                                                              ν12 = 70.2                               r23 = -19.4285                                                                            d23 = variable                                                      r24 = ∞                                                                             d24 = 1.0204                                                                               n13 = 1.51633                                                                              ν13 = 64.2                               r25 = ∞                                                                  ______________________________________                                         Variable Thicknesses of Air Lenses                                                       Focal Lengths                                                                    W            M      T                                              Air Lenses  1.00         3.11   11.41                                          ______________________________________                                         d 7         0.23         3.27   5.30                                           d13         5.49         2.45   0.43                                           d18         1.99         1.30   1.72                                           d23         1.02         1.71   1.30                                           ______________________________________                                    

    ______________________________________                                         Embodiment 4                                                                   f = 1 to 11.41   fno = 1:1.85 to 1.89   2w = 66.3° to                   ______________________________________                                         6.6°                                                                    r 1 = 23.1130                                                                             d 1 = 0.3061                                                                               n 1 = 1.84666                                                                              ν 1 = 23.8                               r 2 = 7.7360                                                                              d 2 = 1.1641                                                                               n 2 = 1.51633                                                                              ν 2 = 64.2                               r 3 = 83.1025                                                                             d 3 = 0.0408                                                        r 4 = 17.3968                                                                             d 4 = 0.6300                                                                               n 3 = 1.72000                                                                              ν 3 = 50.3                               r 5 = -212.2231                                                                           d 5 = 0.0408                                                        r 6 = 5.6836                                                                              d 6 = 0.8489                                                                               n 4 = 1.72000                                                                              ν 4 = 50.3                               r 7 = 16.7357                                                                             d 7 = variable                                                      r 8 = 13.5151                                                                             d 8 = 0.1633                                                                               n 5 = 1.77250                                                                              ν 5 = 49.6                               r 9 = 1.4751                                                                              d 9 = 0.7513                                                        r10 = -4.0496                                                                             d10 = 0.1429                                                                               n 6 = 1.69680                                                                              ν 6 = 55.5                               r11 = 3.0691                                                                              d11 = 0.2044                                                        r12 = 3.0462                                                                              d12 = 0.4043                                                                               n 7 = 1.84666                                                                              ν 7 = 23.8                               r13 = 21.0780                                                                             d13 = variable                                                      r14 = (stop)                                                                              d14 = 0.2449                                                        r15 = 4.6349                                                                              d15 = 0.5607                                                                               n 8 = 1.66672                                                                              ν 8 = 48.3                               r16 = -4.6349                                                                             d16 = 0.0851                                                        r17 = -2.9879                                                                             d17 = 0.2041                                                                               n 9 = 1.68893                                                                              ν 9 = 31.1                               r18 = -5.1198                                                                             d18 = variable                                                      r19 = 4.2375                                                                              d19 = 0.1633                                                                               n10 = 1.84666                                                                              ν10 = 23.8                               r20 = 1.9591                                                                              d20 = 0.5711                                                                               n11 = 1.51633                                                                              ν11 = 64.2                               r21 = -10.7492                                                                            d21 = 0.0306                                                        r22 = 3.8768                                                                              d22 = 0.4255                                                                               n12 = 1.48749                                                                              ν12 = 70.2                               r23 = -14.1764                                                                            d23 = variable                                                      r24 = ∞                                                                             d24 = 1.0204                                                                               n13 = 1.51633                                                                              ν13 = 64.2                               r25 = ∞                                                                  ______________________________________                                         Variable Thicknesses of Air Lenses                                                       Focal Lengths                                                                    W            M      T                                              Air Lenses  1.00         3.10   11.41                                          ______________________________________                                         d 7         0.22         3.28   5.31                                           d13         5.52         2.47   0.44                                           d18         2.12         1.43   1.83                                           d23         1.02         1.72   1.31                                           ______________________________________                                    

    ______________________________________                                         Embodiment 5                                                                   f = 1 to 11.41   fno = 1:1.65 to 1.79   2w = 66.3° to                   ______________________________________                                         6.6°                                                                    r 1 = 17.4772                                                                             d 1 = 0.3061                                                                               n 1 = 1.84666                                                                              ν 1 = 23.8                               r 2 = 7.1092                                                                              d 2 = 1.1085                                                                               n 2 = 1.51633                                                                              ν 2 = 64.2                               r 3 = 28.3027                                                                             d 3 = 0.0408                                                        r 4 = 17.5011                                                                             d 4 = 0.6399                                                                               n 3 = 1.72000                                                                              ν 3 = 50.3                               r 5 = -320.0273                                                                           d 5 = 0.0408                                                        r 6 = 5.9532                                                                              d 6 = 0.8736                                                                               n 4 = 1.72000                                                                              ν 4 = 50.3                               r 7 = 19.3442                                                                             d 7 = variable                                                      r 8 = 13.4358                                                                             d 8 = 0.1633                                                                               n 5 = 1.77250                                                                              ν 5 = 49.6                               r 9 = 1.4947                                                                              d 9 = 0.7873                                                        r10 = -4.2621                                                                             d10 = 0.1429                                                                               n 6 = 1.69680                                                                              ν 6 = 55.5                               r11 = 3.3615                                                                              d11 = 0.2006                                                        r12 = 3.1714                                                                              d12 = 0.3994                                                                               n 7 = 1.84666                                                                              ν 7 = 23.8                               r13 = 23.9889                                                                             d13 = variable                                                      r14 = (stop)                                                                              d14 = 0.2449                                                        r15 = 4.3405                                                                              d15 = 0.5579                                                                               n 8 = 1.58267                                                                              ν 8 = 46.4                               r16 = -5.4173                                                                             d16 = 0.1136                                                        r17 = -3.1313                                                                             d17 = 0.2041                                                                               n 9 = 1.68893                                                                              ν 9 = 31.1                               r18 = -5.1512                                                                             d18 = variable                                                      r19 = 3.9973                                                                              d19 = 0.1633                                                                               n10 = 1.84666                                                                              ν10 = 23.8                               r20 = 1.8419                                                                              d20 = 0.5479                                                                               n11 = 1.51633                                                                              ν11 = 64.2                               r21 = -14.6634                                                                            d21 = 0.0306                                                        r22 = 3.9015                                                                              d22 = 0.4193                                                                               n12 = 1.48749                                                                              ν12 = 70.2                               r23 = -10.8455                                                                            d23 = variable                                                      r24 = ∞                                                                             d24 = 1.0204                                                                               n13 = 1.51633                                                                              ν13 = 64.2                               r25 = ∞                                                                  ______________________________________                                         Variable Thicknesses of Air Lenses                                                       Focal Lengths                                                                    W            M      T                                              Air Lenses  1.00         3.12   11.40                                          ______________________________________                                         d 7         0.20         3.41   5.55                                           d13         5.77         2.56   0.42                                           d18         1.92         1.250  1.65                                           d23         1.02         1.69   1.28                                           ______________________________________                                    

    ______________________________________                                         Embodiment 6                                                                   f = 1 to 11.41   fno = 1:1.66 to 1.76   2w = 66.3° to                   ______________________________________                                         6.6°                                                                    r 1 = 14.8459                                                                            d 1 = 0.3061                                                                               n 1 = 1.84666                                                                               ν 1 = 23.8                               r 2 = 6.8670                                                                             d 2 = 1.1213                                                                               n 2 = 1.51633                                                                               ν 2 = 64.2                               r 3 = 25.7908                                                                            d 3 = 0.0408                                                         r 4 = 14.1586                                                                            d 4 = 0.5805                                                                               n 3 = 1.72000                                                                               ν 3 = 50.3                               r 5 = 59.9705                                                                            d 5 = 0.0408                                                         r 6 = 6.1766                                                                             d 6 = 0.8523                                                                               n 4 = 1.72000                                                                               ν 4 = 50.3                               r 7 = 20.3556                                                                            d 7 = variable                                                       r 8 = 10.6354                                                                            d 8 = 0.1633                                                                               n 5 = 1.77250                                                                               ν 5 = 49.6                               r 9 = 1.4762                                                                             d 9 = 0.8341                                                         r10 = -4.4874                                                                            d10 = 0.1429                                                                               n 6 = 1.69680                                                                               ν 6 = 55.5                               r11 = 3.6681                                                                             d11 = 0.1712                                                         r12 = 3.1466                                                                             d12 = 0.4133                                                                               n 7 = 1.84666                                                                               ν 7 = 23.8                               r13 = 17.8814                                                                            d13 = variable                                                       r14 = (stop)                                                                             d14 = 0.2449                                                         r15 = 5.6434                                                                             d15 = 0.4931                                                                               n 8 = 1.72000                                                                               ν 8 = 50.3                               r16 = -5.6434                                                                            d16 = 0.1199                                                         r17 = -3.0359                                                                            d17 = 0.2041                                                                               n 9 = 1.68893                                                                               ν 9 = 31.1                               r18 = -4.5760                                                                            d18 = variable                                                       r19 = 3.8997                                                                             d19 = 0.1633                                                                               n10 = 1.84666                                                                               ν10 = 23.8                               r20 = 1.9223                                                                             d20 = 0.5267                                                                               n11 = 1.51633                                                                               ν11 = 64.2                               r21 = -17.5418                                                                           d21 = 0.0306                                                         r22 = 4.2561                                                                             d22 = 0.4404                                                                               n12 = 1.48749                                                                               ν12 = 70.2                               r23 = -7.8729                                                                            d23 = variable                                                       r24 = ∞                                                                            d24 = 1.0204                                                                               n13 = 1.51633                                                                               ν13 = 64.2                               r25 = ∞                                                                  ______________________________________                                         Variable Thicknesses of Air Lenses                                                       Focal Lengths                                                                    W            M      T                                              Air Lenses  1.00         3.14   11.40                                          ______________________________________                                         d 7         0.17         3.48   5.68                                           d13         5.80         2.49   0.29                                           d18         1.99         1.36   1.73                                           d23         1.02         1.66   1.28                                           ______________________________________                                    

    ______________________________________                                         Embodiment 7                                                                   f = 1 to 12.01   fno = 1:1.70 to 1.83   2w = 65.8° to                   ______________________________________                                         6.2°                                                                    r 1 = 13.8790                                                                            d 1 = 0.302 n 1 = 1.84666                                                                               ν 1 = 23.8                               r 2 = 6.6375                                                                             d 2 = 1.1117                                                                               n 2 = 1.51633                                                                               ν 2 = 64.2                               r 3 = 22.8527                                                                            d 3 = 0.0404                                                         r 4 = 13.2082                                                                            d 4 = 0.5740                                                                               n 3 = 1.72000                                                                               ν 3 = 50.3                               r 5 = 46.7660                                                                            d 5 = 0.0404                                                         r 6 = 6.1788                                                                             d 6 = 0.8449                                                                               n 4 = 1.72000                                                                               ν 4 = 50.3                               r 7 = 20.8885                                                                            d 7 = variable                                                       r 8 = 10.4085                                                                            d 8 = 0.1617                                                                               n 5 = 1.77250                                                                               ν 5 = 49.6                               r 9 = 1.4575                                                                             d 9 = 0.8270                                                         r10 = -4.4828                                                                            d10 = 0.1415                                                                               n 6 = 1.69680                                                                               ν 6 = 55.5                               r11 = 3.5913                                                                             d11 = 0.1698                                                         r12 = 3.1001                                                                             d12 = 0.4042                                                                               n 7 = 1.84666                                                                               ν 7 = 23.8                               r13 = 17.0862                                                                            d13 = variable                                                       r14 = (stop)                                                                             d14 = 0.2449                                                         r15 = 5.3231                                                                             d15 = 0.4851                                                                               n 8 = 1.72000                                                                               ν 8 = 50.3                               r16 = -5.3231                                                                            d16 = 0.1067                                                         r17 = -3.0365                                                                            d17 = 0.2021                                                                               n 9 = 1.68893                                                                               ν 9 = 31.1                               r18 = -4.7119                                                                            d18 = variable                                                       r19 = 3.9957                                                                             d19 = 0.1617                                                                               n10 = 1.84666                                                                               ν10 = 23.8                               r20 = 1.9037                                                                             d20 = 0.5215                                                                               n11 = 1.51633                                                                               ν11 = 64.2                               r21 = -19.9577                                                                           d21 = 0.0303                                                         r22 = 4.2388                                                                             d22 = 0.4366                                                                               n12 = 1.48749                                                                               ν12 = 70.2                               r23 = -7.9110                                                                            d23 = variable                                                       r24 = ∞                                                                            d24 = 1.0106                                                                               n13 = 1.51633                                                                               ν13 = 64.2                               r25 = ∞                                                                  ______________________________________                                         Variable Thicknesses of Air Lenses                                                       Focal Lengths                                                                    W            M      T                                              Air Lenses  1.00         3.11   11.41                                          ______________________________________                                         d 7         0.16         3.48   5.69                                           d13         5.81         2.49   0.27                                           d18         1.94         1.26   1.73                                           d23         1.01         1.69   1.22                                           ______________________________________                                    

    ______________________________________                                         Embodiment 8                                                                   f = 1 to 11.40   fno = 1:1.65 to 1.79   2w = 66.3° to                   ______________________________________                                         6.6°                                                                    r 1 = 23.1696                                                                            d 1 = 0.3061                                                                               n 1 = 1.84666                                                                               ν 1 = 23.3                               r 2 = 7.7976                                                                             d 2 = 0.9628                                                                               n 2 = 1.51633                                                                               ν 2 = 64.2                               r 3 = 23.2595                                                                            d 3 = 0.0408                                                         r 4 = 13.4142                                                                            d 4 = 0.8380                                                                               n 3 = 1.72000                                                                               ν 3 = 50.3                               r 5 = -61.4809                                                                           d 5 = 0.0408                                                         r 6 = 5.8666                                                                             d 6 = 0.7968                                                                               n 4 = 1.72000                                                                               ν 4 = 50.3                               r 7 = 14.5960                                                                            d 7 = variable                                                       r 8 = 12.2119                                                                            d 8 = 0.1633                                                                               n 5 = 1.77250                                                                               ν 5 = 49.6                               r 9 = 1.5441                                                                             d 9 = 0.7873                                                         r10 = -3.5821                                                                            d10 = 0.1429                                                                               n 6 = 1.69680                                                                               ν 6 = 55.5                               r11 = 3.5821                                                                             d11 = 0.2724                                                         r12 = 3.6512                                                                             d12 = 0.4921                                                                               n 7 = 1.84666                                                                               ν 7 = 23.8                               r13 = 71.9571                                                                            d13 = variable                                                       r14 = (stop)                                                                             d14 = 0.2449                                                         r15 = 3.3555                                                                             d15 = 0.6246                                                                               n 8 = 1.58313                                                                               ν 8 = 59.4                                 aspherical                                                                   r16 = -8.2938                                                                            d16 = 0.2370                                                         r17 = -2.4818                                                                            d17 = 0.2041                                                                               n 9 = 1.59551                                                                               ν 9 = 39.2                               r18 = -2.9772                                                                            d18 = variable                                                       r19 = 4.9603                                                                             d19 = 0.1633                                                                               n10 = 1.78472                                                                               ν10 = 25.7                               r20 = 1.7557                                                                             d20 = 0.5777                                                                               n11 = 1.51633                                                                               ν11 = 64.2                               r21 = -12.4206                                                                           d21 = 0.0306                                                         r22 = 3.9947                                                                             d22 = 0.3996                                                                               n12 = 1.51633                                                                               ν12 = 64.2                               r23 = -12.9695                                                                           d23 = variable                                                       r24 = ∞                                                                            d24 = 1.0204                                                                               n13 = 1.51633                                                                               ν13 = 64.2                               r25 = ∞                                                                  ______________________________________                                         Variable Thicknesses of Air Lenses                                                       Focal Lengths                                                                    W            M      T                                              Air Lenses  1.00         3.10   11.40                                          ______________________________________                                         d 7         0.21         3.43   5.58                                           d13         5.81         2.59   0.45                                           d18         1.61         0.90   1.31                                           d23         1.02         1.71   1.31                                           ______________________________________                                    

In surface r15, the coefficients of the aspherical surface are: a₁ =0.00000D₊₀₀, a₂ =2.52796D₋₀₃, a₃ =2.98987D₋₀₄, a₄ =4.75191D₋₀₄, a₅ =-1.59625D₋₀₄.

                  TABLE 1                                                          ______________________________________                                         Conditional                                                                             Embodiments                                                           Expressions                                                                             1      2      3    4    5    6    7    8                              ______________________________________                                         |M.sub.2 /f.sub.2 |                                                   3.46   3.41   3.22 3.23 3.22 3.22 3.28 3.23                           f.sub.4 /f.sub.w                                                                        4.48   4.42   4.29 4.28 4.31 4.11 4.28 4.71                           f.sub.1 /f.sub.w                                                                        10.99  10.01  8.39 8.43 8.84 9.15 9.10 8.82                           |f.sub.1 /f.sub.2 |                                                   6.14   5.89   5.34 5.36 5.33 5.35 5.39 5.32                           |f.sub.2 /f.sub.w |                                                   1.79   1.70   1.57 1.57 1.66 1.71 1.69 1.66                           f.sub.1 /M.sub.2                                                                        1.77   1.72   1.66 1.66 1.65 1.66 1.64 1.65                           Z/Z.sub.2                                                                               1.39   1.33   1.16 1.16 1.16 1.17 1.13 1.17                           Z.sub.3 /Z.sub.4                                                                        3.40   3.22   1.91 1.89 1.76 1.97 1.56 1.64                           ______________________________________                                    

Constructed as above, the zoom lens systems according to the preferred embodiments of the present invention achieve reduced sizes thereof and wide view angles of 65° or greater.

While the present invention has been described with reference to what are presently considered to be the preferred embodiments, it is to be understood that the invention is not limited to the disclosed embodiments. To the contrary, the invention is intended to cover various modifications and equivalent arrangements included within the spirit and scope of the appended claims. 

What is claimed is:
 1. A rear-focus zoom lens system comprising:a first lens group having a positive refractive power that remains fixed during zooming; a second lens group having a negative refractive power that moves for zooming; a third lens group having a positive refractive power that remains fixed in position during zooming; and a fourth lens group having a positive refractive power that moves for zooming and focusing, said first through fourth lens groups being arranged in that order from the object side, wherein said zoom lens system satisfies the following conditional expression:

    4<f.sub.4 /fw<6,

wherein f₄ is the focal length of the fourth lens group and fw is the focal length of said zoom lens system at a wide-angle end, and further satisfying the following conditional expression:

    7<f.sub.1 /fw<15

wherein, f₁ is the focal length of said first lens group and fw is the focal length of said zoom lens system at the wide-angle end.
 2. A rear-focus zoom lens system according to claim 1, further satisfying the following conditional expression:

    2.8<|M.sub.2 /f.sub.2 |<5

wherein, M₂ is a displacement of said second lens group when the zoom lens system moves from the wide-angle end toward the telephoto end; and f₂ is the focal length of said second lens group.
 3. A rear-focus zoom lens system according to claim 2, further satisfying the following conditional expression:

    8<|f.sub.1 /f.sub.w |<12

wherein f₁ is the focal length of said first lens group and f_(w) is the focal length of said zoom lens system at the wide-angle end.
 4. A rear-focus zoom lens system according to claim 1, further satisfying the following conditional expression:

    4.0<|f.sub.1 /f.sub.2 |<10.0

wherein f₁ is the focal length of said first lens group and f₂ is the focal length of said second lens group.
 5. A rear-focus zoom lens system according to claim 1, further satisfying the following conditional expression:

    1.55<|f.sub.1 /f.sub.w |<1.8

wherein f₁ is the focal length of said first lens group and f_(w) is the focal length of said zoom lens system at the wide-angle end.
 6. A rear-focus zoom lens system according to claim 1, further satisfying the following conditional expression:

    1.0<f.sub.1 /M.sub.2 <1.85

wherein f₁ is the focal length of said first lens group and M₂ is a displacement of said second lens group when the zoom lens system moves from the wide-angle end toward the telephoto end.
 7. A rear-focus zoom lens system according to claim 1, further satisfying the following conditional expression:

    1.0<z/z.sub.2 <1.5

wherein z is the ratio between the focal length of the entire lens system at the wide-angle end and the focal length of the entire lens system at the telephoto end, and z₂ is the ratio between the magnifications at the wide-angle end and the telephone end in said second lens group.
 8. A rear-focus zoom lens system according to claim 1, further satisfying the following conditional expression:

    1.65<z.sub.3 /z.sub.4 <4.0

wherein z₃ is the ratio between the magnifications at the wide-angle end and the telephoto end in said third lens group, and z₄ is the ratio between the magnifications at the wide-angle end and the telephoto end in said fourth lens group.
 9. A rear-focus zoom lens system according to claim 1, wherein said first lens group comprises, from the object side, a combined lens, which consists of a negative lens and a positive lens, and at least one positive lens.
 10. A rear-focus zoom lens system according to claim 1, wherein said second lens group comprises, from the object side, a negative meniscus lens having a substantially concave surface toward the image side, a biconcave lens, and a positive lens having a concave surface toward the image side.
 11. A rear-focus zoom lens system according to claim 1, wherein said third lens group comprises, from the object side, a biconvex positive lens having an aspherical surface and a negative lens having a concave surface toward the object side.
 12. A rear-focus zoom lens system according to claim 1, wherein said fourth lens group comprises, from the object side, a negative meniscus lens concave toward the image side, and at least one positive lens.
 13. A rear-focus zoom lens system according to claim 1, wherein said zoom lens system satisfies the following conditional expression:

    4.11<f.sub.4 /fw<6,

wherein f₄ is the focal length of the fourth lens group and fw is the focal length of said zoom lens system at a wide-angle end.
 14. A rear-focus zoom lens system comprising:a first lens group having a positive refractive power that remains fixed during zooming; a second lens group having a negative refractive power that moves for zooming; a third lens group having a positive refractive power that remains fixed in position during zooming; a fourth lens group having a positive refractive power that moves for zooming and focusing, said first through fourth lens groups being arranged in that order from the object side, wherein said zoom lens system satisfies the following conditional expression:

    4<f.sub.4 /fw<6,

wherein f₄ is the focal length of the fourth lens group and fw is the focal length of said zoom lens system at a wide-angle end; and wherein said zoom lens system further satisfies the following conditional expression:

    2.8<|M.sub.2 /f.sub.2 |<5

wherein, M₂ is a displacement of said second lens group when the zoom lens system moves from the wide-angle end toward the telephoto end, and f₂ is the focal length of said second lens group. 